This is an interesting conversation. I'll throw in another opinion on the debate that hasn't really been brought up yet.
I actually prefer 4p games for 2 reasons:
I am very social, and the more friends around the better;
I find 2p Dominion (and many other 2p versions of games) to get boring. I know many forum members will jump all over me for saying that, but please read my explanation before expressing your distaste for my opinion. I am not saying 2p is bad or uninteresting, but rather gets stale when you consider that you are constantly looking for the "right move" or "best play" based on the math, research, simulations, etc. Now Dominion is one of the best games at avoiding thia staleness since every time you play it's with a different set of cards, but once you've become accustomed to playing 2p, it just becomes more about who can make the "right" buys first, or who understands the math better. Compare that to 4p, where there are far more variables to consider. Yes, you lose some of the skill game needed for 2p, but does that really classify it as "bad". I find it much more fun when I can't just follow a set of rules of what cards to buy and what not to buy. With 3 other players to consider, the "right" buy is not as clearcut. Again, 2p and 4p play very differently, but I don't think you can classify either as "bad".
Also, when my group plays 4p, we actually don't socialize that much, we are usually watching each other take our turns because they are usually interesting since it's not the same old big money or engine strategy every time. We play the game for fun! Not saying that the 2p skill game isn't fun, I do indeed still enjoy playing it, but I find I have more fun when the game is unpredictable.